Many signals of interest have bandwidths that are much larger than the bandwidth of the test equipment that must be utilized to measure the signals. For example, the bandwidth of a conventional oscilloscope is significantly less than that of many RF signals used in communication systems and the like. Hence, measuring such signals on an oscilloscope presents a problem. A sampling oscilloscope circumvents this problem for repetitive signals by utilizing a sampling circuit that measures the signal over a very brief time interval and displays the resulting sample as one point of a graph. Typically, one sample is taken during each period of the repetitive signal. The time of the sample relative to the beginning of the signal repetition is varied in each period such that successive points sample the signal at different points relative to the beginning of each period of the signal. Hence, the collection of samples can be displayed to provide a conventional display of voltage as a function of time.
Only a small fraction of the energy in the signal is extracted at each sample. The amount of energy that is extracted depends on the time interval over which the sampling window is opened. The sampling window must be of sufficiently short duration that changes in the signal amplitude during the sampling interval can be ignored. Hence, as the bandwidth of the signal being measured increases, the sampling time interval must be decreased, leading to still further decreases in the amount of energy that is extracted from the signal. The ratio of this sample energy to the noise in the instrument determines the signal-to-noise ratio of the instrument. Higher signal-to-noise ratios provide more accurate measurements, and hence, are preferred.
One possible method for increasing the signal-to-noise ratio of the sampled system is to increase the repetition rate of the sampling. However, an increase in the sampling rate is not always practical, because the electronics following the sampler also have bandwidth limitations.
Another method for improving the amount of energy extracted from the signal is to utilize a mixer in which a sinusoidal local oscillator signal is used to extract energy from the signal. However, this approach is only practical for signals having limited bandwidth such as a modulated RF carrier in which the carrier is removed so that the modulation waveform can be examined.